It is known to use aluminum both as a deoxidizing agent and as an alloy additive to steel. Aluminum is a very reactive metal with oxygen-containing atmospheres at elevated temperatures in that it reacts uncontrollably and exothermically with oxygen. The free energy of reaction is very high. Thus, the reaction is difficult to control once it starts, although it is this property which is used to deoxidize iron or steel when controlled properly. The terms "iron" or "steel" used herein are used interchangeably. Whenever one term is used, it includes the other.
As a deoxidizing agent, aluminum per se is very inefficient as much of it is lost during submersion into the steel bath. Because of its low density, it remains near the surface of the bath and tends to react with the furnace atmosphere which contains some oxygen and also with the slag. The reaction is limited to a small area unless some stirring means is employed. However, even this has its limitations.
Thus, it has been the practice to add aluminum as a ferro-aluminum alloy. The alloy having a higher density than aluminum per se penetrates the steel bath more easily when emersed and does not react as rapidly and thereby provides better control of deoxidation or alloying, if alloying is the objective.
Generally speaking, the composition of commercially available ferro-aluminum alloy is nominally 35 to 40% aluminum and 65% to 60% iron or steel. However, this alloy has poor storage stability in that it tends to decrepitate, crumble and fall apart. While such pieces can be and have been used to deoxidize steel, they are difficult to handle and to provide efficient submersion into the steel bath. In addition, the pieces or chunks tend to break down further due to thermal shock when added to the steel bath which has an adverse affect on the efficiency of deoxidation.
To avoid the foregoing, the ferro-aluminum alloys employed contained low amounts of aluminum ranging from about 25% to 30% by weight, since these alloys maintain their structural integrity during storage.
Recently a patent has issued (U.S. Pat. No. 4,801,328) which attempts to overcome the foregoing problem by going back to the use of substantially pure aluminum, but using it in a different way. According to the patent, a deoxidizing agent is provided which utilizes a core of material which is chemically non-reactive with molten ferrous metal (e.g., a core of iron) and which has a density substantially greater than that for aluminum. Using the core as a substrate, it is completely covered by one or more discrete layers of metallic aluminum to provide an average composition ranging from 30% to 50% Al. Because of the use of metallic aluminum, the addition agent has good integrity and does not decrepitate or crumble.
The aluminum used is substantially pure. According to the patent, it is preferably maintained at a purity level of 95% or higher of aluminum. The patent reports improved deoxidation efficiency when the deoxidizing agent of the invention is used. However, from the data reported, the improvement isn't that much better than the use of conventional ferro-aluminum alloy alone.
A disadvantage of the patented invention is that metallic aluminum used has a very low melting point of about 659.degree. C. compared to iron or steel which is about 1530.degree. C. Because of this, the aluminum tends to melt substantially immediately and therefore tends to separate from the core which does not allow sufficient time to carry out the reaction without using excess deoxidizing agent.
Applicant has discovered a method of using ferro-aluminum alloy itself over the effective range of about 25% to 55% aluminum and the balance iron which will withstand storage and not decrepitate or crumble.